In propeller driven marine vehicle applications, it is critically important to properly match power, thrust, and speed requirements with an appropriately designed propeller. The performance of a particular propeller depends upon many parameters including the number of blades, the propeller diameter, the blade area, and the geometric pitch of the propeller. Of these parameters, geometric pitch, or the theoretical axial advance of a propeller through the water per single blade revolution, has traditionally been somewhat cumbersome if not difficult to determine.
Prior to the instant invention, measurement of geometric pitch often required dismounting the propeller from its shaft and vehicle and measurement of pitch by means of bulky, semi-portable and often elaborate instruments. Some prior art pitch measuring devices are shown in U.S. Pat. Nos. 1,547,380 to Godfrey; 1,696,525 to Cooledge; 2,132,407 to Fowler; 2,172,368 to Eby; 2,248,973 to Eby; 2,383,527 to Whitechester; and 2,421,754 to Little et al.
It has been determined that the geometric pitch for a given propeller is a constant distance which distance depends directly upon the average blade angle or angle of attack of the propeller. In a properly designed propeller the angle of attack for each blade element diminishes as the radial distance from the axis of rotation of the propeller increases. This gradual decrease in blade angle gives the propeller blade its twisted appearance. For a propeller having a given pitch, a given angle of attack will exist at only one radial distance (r) from the axis of rotation of the propeller. From this premise, the following relationship between pitch, angle of attack, and distance from the center of rotation has evolved: EQU P = 2 .pi.r tan .alpha.
Where P = the geometric pitch of the propeller;
2 .pi. = a constant; PA1 r = the radial distance from the axis of rotation to the point of measurement; and PA1 .alpha. = the angle between a line drawn tangent to the pressure face of a blade and the plane of rotation of the propeller.